This application claims the benefit of U.S. Provisional Application No. 60/018,124, filed May 22, 1996, the disclosure of which is incorporated herein by reference.
The present invention relates, in general, to a sensor for determining internal pressure in a container having a flexible wall, and more particularly to a container inspection system which includes a sensor for measuring the shape of a container wall or for determining the resonant vibration characteristics of the wall to provide an indication of the pressure or vacuum within the container.
It is frequently essential to inspect containers of various kinds, and particularly food and beverage containers, to determine whether they have the desired internal pressure, since insufficient vacuum or a positive internal pressure may indicate a leaky container, insufficient evacuation, improper ingredients, or food spoilage. Furthermore, aerosol cans need to be checked to see if they contain sufficient positive internal pressure, and carbonated beverage cans need to be measured to see whether the contents have gone flat due to leakage, and a variety of methods and devices have been developed for this purpose. One such device is described in U.S. Pat. No. 3,802,252, assigned to the assignee of the present application, which discloses a pressure and vacuum monitoring system generally referred to as an electromagnetic pulse system. This device utilizes a transducer coil and means for discharging an electrical pulse through the coil to produce a magnetic field pulse which resiliently deflects an adjacent flexible wall of a container under test. This deflection causes the wall of the container to vibrate and to produce an acoustical response which is a function of the internal pressure in the container. Any change in the internal pressure usually changes the tension on the container wall, causing the resonant frequency and amplitude of the acoustic response to change. The acoustic energy of the vibrating wall is detected with a microphone and its frequency and amplitude content are analyzed to distinguish failed containers from good containers. The wall selected for measurement is usually the end wall of a metal can, or is the metal cap or lid for other containers such as plastic or glass jars or bottles.
Although the electromagnetic pulse system has been found to be useful and reliable for many container applications, it may not always be effective in discriminating between good and bad containers, particularly in three different situations. First, it has been discovered that a can end or container lid may generate similar acoustic responses at two different internal pressures, one under vacuum (or a negative pressure) with the container wall curved inwardly, and the other under a positive pressure with the wall curved outwardly. Because of this, the testing may be unacceptably ambiguous, for one condition is usually acceptable while the other is not. Second, the amplitude of the acoustic response of a container may be attenuated by a case or carton used to package a cluster of the containers. In such a situation, there may be an insufficient signal-to-noise-ratio to provide reliable discrimination between containers with acceptable and unacceptable internal pressures, or to provide sufficiently low false reject rates. Third, acoustically noisy background environments may degrade the signal-to-noise ratio, also causing unreliable bad versus good container discrimination or unacceptable false reject rates.
To overcome the foregoing problems, the system of U.S. Pat. No. 4,177,718, also assigned to the assignee of the present application, was developed. In this second system, a passive sensing approach was adopted. Three inductive coil sensors were arrayed to sense the passing of a pressurized container by detecting changes in an inductive magnetic field, thereby to measure the amount of deflection of the container flexible wall as a function of internal pressure. However, these sensors generated a continuously changing magnetic field rather than a magnetic impulse. Thus, in accordance with the '718 patent, the instantaneous distance between the ends of at least two sensing coils and the surface of a can wall directly beneath the sensing coils was measured. The profile of the top of the can was then estimated by comparing the sensing coil measurements.
One problem with this second system was that an inductive distance sensor must be in relatively close proximity to the container wall in order to make an accurate measurement, for inductive sensors are only useful within a maximum "standoff" range, or distance to the container wall, of approximately one quarter of an inch. As a result, it was very difficult to sense the internal pressure of containers which were packed in a closed case or carton, for example, for such closed cases often have more than one layer of corrugated cardboard or the like between the can end and the sensor, thus exceeding the maximum standoff range. An excessive standoff range can also be encountered when the containers have settled in the case.
A second problem with the inductive sensor system is that it is not always sensitive enough to measure small changes in the shape of a container lid or wall, even though there is an unacceptable change in pressure. This situation occurs when the container wall is relatively stiff and does not change shape very much with changing pressure. For this type of container, the use of changes in vibration resonant frequency and amplitude as the discriminating characteristic frequently provides better results.
Thus, a different sensor type is required to address the problems of insufficient standoff range and weak acoustic signal reception in inspecting the internal pressure of containers.